The invention relates to a method in connection with an inverter, the inverter comprising a direct voltage intermediate circuit, an optimum switching table and output power switches.
A frequency converter is a device that is typically used for motor control. A frequency converter typically consists of two converters, between which there is a direct voltage or direct current intermediate circuit. The converters of a frequency converter can be implemented in such a way that they are capable of functioning only as rectifiers, or in such a way that they can function, if required, as both rectifiers and inverters. An example of rectifiers is a diode bridge, and an example of a converter applicable to both rectification and inversion is a converter bridge implemented by means of transistors. An inverter is typically used to control the power transferred from the intermediate circuit of a frequency converter to a motor. By means of an inverter, motor control can be implemented reliably in such a way that the motor implements for example the desired speed or torque command accurately. An inverter can also be used for controlling the power flow from the electrical power network to the intermediate circuit of the frequency converter. An inverter used for this purpose is usually called a network inverter. A network inverter allows efficient control of the active and reactive powers transferred between the electrical power network and the frequency converter.
A network inverter is used for replacing a diode bridge rectifier of a frequency converter, particularly in such objects of use where it is desirable to invert the braking energy of a motor back to the electrical power network. The curve form of the supply current of the network inverter can be made very sinusoidal, owing to which it is well applicable to objects where the lowering of the electricity quality caused by the frequency converter must be reduced.
Present high-rate power semiconductor components and signal processors makes it possible to implement the control of an inverter dynamically with a high rate by using solutions based on direct torque control (DTC). A known network inverter based on DTC control is shown in the block diagram of FIG. 1. In the solution of FIG. 1, the switching commands of a semiconductor power switch bridge is formed in accordance with the DTC principle on the basis of the absolute value of the flux linkage vector and of the torque by using a DTC block 13. The computational flux linkage vector of the network inverter is calculated with a voltage integral{overscore (ψ)}=∫ūdt,  (1)
and the torque proportional to the power is calculated by the cross product of the power vector and the flux linkage vectorte=|{overscore (ψ)}×ī|.  (2)
An intermediate circuit voltage controller 11 generates the torque reference te,ref on the basis of the difference between the measured intermediate circuit voltage and intermediate circuit voltage reference. The absolute value reference |{overscore (ψ)}|ref of the flux linkage is generated by means of a reactive power controller 12 by comparing the estimated reactive power qest and reactive power reference qref. In connection with network inverters, a low pass filter 14 is typically used between the inverter and the network. When the filter type is an L filter, the reactive power is estimated with the equationqest=({overscore (ψ)}v·ī)ω,  (3)where ω is the electric angular frequency corresponding to the direct wave of the network, and {overscore (ψ)}v is the flux linkage vector of the network. The flux linkage vector of the network is estimated with the equation{overscore (ψ)}v={overscore (ψ)}−L ī,  (4)where L is the inductance of the network filter.
Conventionally, the object of application of power vector control methods has been control of electric motor use provided with separate PWM modulators. The principle of power vector control of electric motor use provided with a PWM modulator is shown in FIG. 3. Power controllers 31, 32 generate a voltage vector reference, the intention being to implement the reference by means of a PMW modulator 33 controlling power switches 34. The solution is disclosed for instance in the publication Harnefors, L., ‘On Analysis, Control and Estimation of Variable-speed drives’, Doctoral dissertation, Part I, page 44, Royal Institute of Technology, Stockholm, Sweden, 1997.
A direct DTC control method of a network inverter similar to that in FIG. 1 does not actively control the currents of the converter. As a result, the currents of the converter may be non-sinusoidal and contain significantly lower harmonic components, such as 5th or 7th harmonic. The harmonic current components supplied by the converter are particularly intensified if, instead of an L filter, the network filter is an LCL filter.